Printing plates containing radiation-sensitive compositions are well known. Such printing plates are imageable by infrared and/or UV/visible radiation. As a result of such radiation, the exposed area of a positive plate becomes more soluble in the developer than the non-exposed area (in a negative printing plate, the reverse is true).
Prior art examples disclosing radiation-sensitive compositions, and printing plates containing same, include U.S. Pat. Nos.: 4,708,925; 5,085,972; 5,286,612; 5,372,907; 5,372,915; 5,441,850; 5,466,557; 5,491,046; 5,340,699; and 5,466,557; and European Patent Application 0 672 954 A2.
U.S. Pat. No. 5,491,046 is an example of a printing plate containing a radiation-sensitive composition wherein the composition is comprised of a resole resin; novolac resin; latent Bronsted acid; and infrared absorber. However, the resin for such radiation-sensitive composition requires the combination of a resole and novolac resin. If, for example, the novolac resin is omitted, the image/exposure step will not provide a useful means of imaging, since subsequent contact with the developer will remove the coating from both the exposed and unexposed areas. Consequently, printing plates processed in such manner are not useful.
Moreover, these prior art printing plates require the use of highly alkaline developers which are prone to reaction with atmospheric carbon dioxide, thereby necessitating the use of sealed, specialized equipment. The prior art radiation-sensitive compositions also entail the use of low molecular weight, promoters, which may volatilize.
Printing plates containing the radiation-sensitive compositions of the present invention have several distinct advantages over printing plates containing the prior art compositions. In particular, the radiation sensitive composition for the printing plates of the present invention are not restricted to a particular resin system; are imageable by infrared or ultraviolet radiation sources; and employ developers that are easier to use. Therefore, the radiation-sensitive composition of the present invention provide greater processing latitude (e.g. temperature, dwell time, etc.) and a greater degree of flexibility in customizing the printing plates to meet the particular needs of each printer.
Finally, printing plates, containing the radiation-sensitive compositions of the present invention, have significantly greater press life (printing runs in excess of 100,000 copies) if not post-baked when compared with non-post-baked prior art printing plates. Such difference in press life is maintained when the plates are post-baked (as a general rule, post-baking extends the press life of printing plates).